When a seven-year-old boy trod on a heroin syringe discarded in a playground, his parents insisted that the police in the sleepy German spa town of Gerolstein open an investigation.

Detectives examined the syringe for DNA traces and deposited the information in a central data bank. That was six years ago and no one expected anything to come of it; the boy has long since forgotten the incident and the anger of the parents has ebbed.

Yet now the addict’s genetic fingerprint has become an essential piece of a puzzle that, if solved, could trap a serial killer. The police this week intensified their search for a woman, dubbed the Phantom of Heilbronn, whom they believe took part in the coldblooded murder of a woman police officer and the maiming of a male colleague at the end of April. Traces of her DNA, identical to that found on the syringe and dozens of other crime scenes, were found in the patrol car where the two officers were shot from behind.

For 14 years this spectral figure has been killing and burgling houses across southern Germany, France and Austria. Her identity has been a mystery, baffling detectives who can see no motive for the three murders committed so far. They can find no witness, they have no name and no face. All that the Phantom of Heilbronn leaves behind is a flake of skin on a revolver, the saliva on a nibbled biscuit. Enough, now, to draw up a crime map showing her apparently haphazard path through three countries, but not enough to produce even the semblance of a portrait.

In charge of hunting the Phantom is Jürgen Brauer, 50, a state prosecutor who has been puzzled by the woman since she began her crime spree in 1993. Then, her DNA was found on the rim of a floral-printed tea cup at the home of Lieselotte Schlenger, 62. She had been killed with wire used to bind bouquets of flowers. When the DNA from the murder of the police officer in Heilbronn was matched to that found in Mrs Schlenger’s apartment, Mr Brauer was stunned: “I just couldn’t believe that the same woman could be capable of these two crimes.”

The Phantom’s genetic fingerprint was found at the scene of a third murder – the strangling of an antiques dealer, Jozef Walzenbach, 61, in Freiburg in 2001. Cash was missing from Mr Walzenbach’s home but there was no further clue to the killer’s motive.

As for the police officers, they had been snacking in the patrol car. The woman and an accomplice must have crept into the back seat and shot them at close range; the officers’ handcuffs were stolen but their wallets were untouched.

Now, thanks to the DNA matches, the police can make some sense of one of the most intriguing detective puzzles since German reunification. The DNA on the syringe suggests that the Phantom is a heroin addict. That could explain some of the apparently random burglaries. On New Year’s Day 2003, the Phantom broke into an office and stole nothing but a coffee tin full of loose change. It was a professional job, leaving no conventional fingerprints, for low gains. Only a minute scraping of her skin betrayed her.

“It is very difficult indeed not to leave a genetic clue,” says Chief Superintendent Horst Haug, who has been involved in the investigation.

The murdered policewoman, Michele Kiesewetter, was an undercover agent trying to expose drug dealers in Heilbronn. Could the Phantom have recognised her and feared that the net was closing around her own drug-dealing activities?

One of the Phantom’s DNA souvenirs was found on a bullet in a gun used in a shoot-out between two gypsy chieftains in May 2005. It seems that she loaded the revolver. The police suspect that the Phantom was active in a gypsy drug racket.

Certainly there was a Romany camp close to the shooting in Heilbronn and the town has a regular coach service to Romania. One bus left Heilbronn shortly after the murder.

“We are concentrating hard on this community,” said an investigator. But no one claims to have seen or heard anything.

Police have begun a nation-wide search and nothing galvanises any force more than the killing of a colleague. But they are tapping in the dark. The authorities discourage ethnic profiling of suspects using DNA samples, worried about comparisons with the Gestapo. But now both police and forensic scientists argue that this is absurd – establishing ethnic origins from the genetic code could at least help to establish the likely colour of the suspect’s hair and eyes. “The English can even identify red heads,” says Bernd Brinkmann, head of forensic medicine at the University of Münster.

The limits of DNA detection have become a pressing issue in the police establishment as old cases are being solved using new genetic testing methods.

But DNA clues alone may not be enough to capture the Phantom. “This is going to call for a lot of shoe-leather,” said one investigator.

“We may not know how big she is, or what she looks like, but we do know that she must have had accomplices. That could be how we bring her in – old-fashioned police work.”

The trail that never goes cold

—In 2005 a US court sentenced Mike Dossett to 40 years in jail for the rape and murder of Rachel Kosub, 22 years after she was found strangled in San Antonio, Texas. Dossett’s DNA matched a sample from the crime scene

—Dandruff found on a stocking mask led Andrew Pearson to be sentenced in 2004 to 15 years in jail for an armed robbery at a Hull caravan company in 1993

—John Wood was arrested in 2001 after stealing £10 of groceries. A DNA sample matched him to a sex attack on two girls aged 9 and 11, at their home in Canterbury, Kent, in 1988. He pleaded guilty to rape and indecent assault and was sentenced to 15 years

—Brian Lunn Field was jailed for life in 2001 after his DNA, taken in 1999 after he was stopped for drink-driving, was matched to a sample from the murder in 1968 near Leatherhead, Surrey, of 14-year-old Roy Tutill

Sources: Home Office, San Antonio Police Department, Yorkshire Post

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